I see this all the time....
It's kind of like ''Yesterday,Today&Tomorrow'' the way one will open a certain color and then fade to other colors in subsequent days~
Keep them deadheaded!

I agree....the flowers undergo an evolution....but you have more than one plant in the picture you showed.
Check again.
Linda C

Kim,

You are right that one zinnia plant can have different colored blossoms. The heirloom zinnia, Exquisite is an example. However, I agree with Linda that it looks very much like your picture contains more than one plant.

Zinnias can even have more than one color on each petal. I have been experimenting with crossing different zinnias with each other, and this is one of my zinnias that has white at the base of each petal.
{{gwi:22027}}
This zinnia has a soft pink and yellow on each petal:
{{gwi:22028}}
This zinnia's petals are orange with a lavender base:
{{gwi:22029}}
I have used Whirligig in some of my crosses and some of my hybrids show the effects of that.

MM

I pictured just one plant, I double checked at the base even though I knew I had planted just one in that spot and watched it grow from a seedling. It gradually sprawled out sideways. To the left of it is a Columbine. I'll remember to deadhead it, and thanks for solving this little mystery.

Kim,

OK, I take your word for it. I was mistaken, and I wasn't counting the Columbine foliage as a second zinnia plant. That's a very sprawling zinnia plant, then. Which could be a good thing. You certainly do have a diversity of flowers on that plant.

MM

Kim,

You are right....I had the same thing happen to me last year and this year. When I was planting my seeds, I remember putting a few (4-5) together in one and when they grew they looked exctly like that. I have one this year too which has red, pink, salmon and light pink all on one plant. I think it looks amazing!!!

Enjoy the marvels of nature.